Refining the perfusion-diffusion mismatch hypothesis

Reliability of MR perfusion-weighted and diffusion-weighted imaging mismatch measurement methods

BACKGROUND AND PURPOSE

We investigated 2 methods of measuring MR imaging perfusion-diffusion mismatch to determine whether reliability is improved by direct measurement on a single, blended map.

MATERIALS AND METHODS

Image software was used for measurement of lesion volumes from diffusion-weighted images (DWI) and mean transit time (MTT) calculated from perfusion-weighted (PWI) images on 64 patients with acute stroke. For the first method, the DWI and MTT lesions were measured separately. For the second method, the mismatch volume was measured directly on the blended images created from the registered DWI and MTT images.

RESULTS

Test-retest agreement was 100% and 97% for the separate and blended methods using mismatch cutoffs of 20% or more versus less than 20%. There were no significant differences in the mismatch statistics between the methods.

CONCLUSIONS

Mismatch volumes by a single reader can provide highly reliable and consistent results even when separately measuring DWI and MTT lesions. Propagation of measurement error was not demonstrated, and the methods were statistically comparable.

3-Tesla versus 1.5-Tesla Magnetic Resonance Diffusion and Perfusion Imaging in Hyperacute Ischemic Stroke

BACKGROUND

Clinical 3-tesla magnetic resonance imaging systems are becoming widespread. No studies have examined differences between 1.5-tesla and 3-tesla imaging for the assessment of hyperacute ischemic stroke (<6 h from symptom onset). Our objective was to compare 1.5-tesla and 3-tesla diffusion and perfusion imaging for hyperacute stroke using optimized protocols.

METHODS

Three patients or their surrogate provided informed consent. Diffusion-weighted imaging (DWI) and perfusion-weighted imaging (PWI) was performed sequentially at 1.5 T and 3 T. DWI, apparent diffusion coefficient (ADC) maps and relative time-to-peak (TTP) maps were registered and assessed. DWI contrast-to-noise ratio (CNR) and ADC contrast were measured and compared. The infarct lesion volume (ILV) and thresholded ischemic volume (TIV) were estimated on the ADC and TTP maps, respectively, with the penumbral volume being defined as the difference between these volumes.

RESULTS

Qualitatively, the 3-tesla TTP images exhibited greater feature detail. Quantitatively, the DWI CNR and ILV were similar at both field strengths, the ADC contrast was greater at 3 T and the TIV and penumbral volumes were much smaller at 3 T.

CONCLUSIONS

Overall, the 3-tesla diffusion and perfusion images were at least as good and in some ways superior to the 1.5-tesla images for assessing hyperacute stroke. The TTP maps showed greater feature detail at 3 T. The ischemic and penumbra volumes were much greater at 1.5 T, indicating a potential difference in the diagnostic utility of the PWI-DWI mismatch between field strengths.

Patients' and bystanders' awareness of stroke and pre-hospital delay after stroke onset: perspectives for thrombolysis in West Pomerania Province, Poland

OBJECTIVE

Thrombolysis is a time-dependent therapy. It is therefore very important how fast stroke patients can reach hospital. The present study was designed to assess which proportion of patients with stroke (from the population of Szczecin, the capital of West Pomerania Province, Poland) reaches hospital within the recommended time from the thrombolytic therapy point of view. The purpose of our study was also to elucidate which factors can influence the time before the ambulance service is called.

PATIENTS AND METHODS

The study involved 1,015 patients with stroke admitted to the Emergency Department of the University Hospital, Szczecin.

RESULTS

235 patients (23.1%) were admitted to the hospital within the appropriate period for thrombolytic therapy. Hospital arrival time was significantly earlier in older patients and in patients with severer neurological deficits. We also observed a tendency for faster hospitalization of women, the highly educated, and patients regularly using antiplatelet drugs for cardiovascular disease prevention.

CONCLUSIONS

The percentage of stroke patients hospitalized within 2 h after stroke onset should be increased. The most susceptible subpopulations in our country seem to be older patients, the highly educated and patients regularly using antiplatelet drugs for cardiovascular disease prevention.

Magnetic resonance perfusion diffusion mismatch and thrombolysis in acute ischaemic stroke: a systematic review of the evidence to date

BACKGROUND

The mismatch between perfusion and diffusion lesions on magnetic resonance perfusion-weighted imaging (PWI)/diffusion-weighted imaging (DWI) may help identify patients for thrombolysis. Evidence underlying this hypothesis was assessed.

METHODS

All papers describing magnetic resonance PWI/DWI findings in patients with acute ischaemic stroke, and their functional and/or radiological outcome at 1 month, with or without thrombolysis were systematically reviewed.

RESULTS

11 papers fulfilled the inclusion criteria. Among these, there were 5 different mismatch definitions and at least 7 different PWI methods. Only 3 papers including 61 patients with and 18 without mismatch provided data on mismatch, outcome and influence of thrombolysis. Mismatch (v no mismatch) without thrombolysis was associated with a non-significant twofold increase in the odds of infarct expansion (odds ratio (OR) 2.2, 95% confidence interval (CI) 0.34 to 14.1), which did not change with thrombolysis (OR 2.0, 95% CI 0.37 to 10.9). Half of the patients without mismatch also had infarct growth (with or without thrombolysis). No data were available on functional outcome.

CONCLUSIONS

Standardised definitions of mismatch and perfusion are needed. Infarct growth may occur even in the absence of mismatch. Currently, data available on mismatch are too limited to guide thrombolysis in routine practice. More data are needed from studies including patients with and without mismatch, and randomised treatment allocation, to determine the role of mismatch.

Acute Ischemic Stroke Lesion Measurement on Diffusion-weighted Imaging–Important Considerations in Designing Acute Stroke Trials With Magnetic Resonance Imaging

BACKGROUND

In acute ischemic stroke, magnetic resonance diffusion-weighted imaging (DWI) is increasingly used to select patients for inclusion or as a surrogate outcome marker in clinical trials, or in routine practice. Little is known of what factors might affect DWI lesion size measurement. We examined morphologic factors that might affect DWI lesion measurement.

METHODS

On DWI obtained less than 24 hours after stroke, we categorized lesions according to DWI appearance (solitary or multifocal; well-defined or ill-defined edges), lesion size (</>5 cm(3)), and time to imaging (<6, 6-12, and 12-24 hours). Two observers (senior neuroradiologist; less-experienced imaging neuroscientist) measured all lesions. In 4 representative cases we assessed DWI lesion volume using two apparent diffusion coefficient thresholds (0.55 and 0.65 x 10(-3) mm(2)/s).

RESULTS

Among 63 patients (33% imaged < 6 hours after stroke), the neuroradiologist measured larger lesion volumes than the imaging neuroscientist (median 4.29 v 3.50 cm(3), respectively, P < .01). Differences between observers were greatest in patients scanned within 6 hours of stroke, in multifocal ill-defined or large lesions (all P < .01). Both apparent diffusion coefficient thresholds underestimated lesion extent and included remote normal tissue, particularly in multifocal ill-defined large lesions.

CONCLUSION

DWI lesion characteristics influence lesion volume measurement. Large, multifocal, ill-defined DWI lesions obtained in less than 6 hours have the greatest variability. Trials using DWI should account for this in their study design.

Experimental treatments for acute ischaemic stroke

Treatments for acute ischaemic stroke continue to evolve. Experimental approaches to restore cerebral perfusion include techniques to augment recanalising therapies, including combination of antiplatelet agents with intravenous thrombolysis, bridging therapy of combining intravenous with intra-arterial thrombolysis, and trials of new thrombolytic agents. Trials with MRI selection criteria are underway to expand the window of opportunity for thrombolysis. Sonothrombolysis and novel endovascular mechanical devices to retrieve or dissolve acute cerebral occlusions are being tested. Approaches to improve cerebral perfusion with other devices and induced hypertension are also being considered. Although numerous neuroprotective agents have not shown benefit, trials of hypothermia, magnesium, caffeinol, high doses of statins, and albumin are continuing. The findings of these randomised trials are anticipated to allow improved treatment of patients with acute stroke.

Can angiogenesis be exploited to improve stroke outcome? Mechanisms and therapeutic potential

Recent developments in our understanding of the pathophysiological events that follow acute ischaemic stroke suggest an important role for angiogenesis which, through new blood vessel formation, results in improved collateral circulation and may impact on the medium-to-long term recovery of patients. Future treatment regimens may focus on optimization of this process in the ischaemic boundary zones or 'penumbra' region adjacent to the infarct, where partially affected neurons exposed to intermediate perfusion levels have the capability of survival if perfusion is maintained or normalized. In this review, we present evidence that angiogenesis is a key feature of ischaemic stroke recovery and neuronal post-stroke re-organization, examine the signalling mechanisms through which it occurs, and describe the therapeutic potential of treatments aimed at stimulating revascularization and neuroprotection after stroke.

Right hemispatial neglect: frequency and characterization following acute left hemisphere stroke

The frequency of various types of unilateral spatial neglect and associated areas of neural dysfunction after left hemisphere stroke are not well characterized. Unilateral spatial neglect (USN) in distinct spatial reference frames have been identified after acute right, but not left hemisphere stroke. We studied 47 consecutive right handed patients within 48h of left hemisphere stroke to determine the frequency and distribution of types of right USN using cognitive testing and MRI imaging. The distribution of USN types was different from the previously reported distribution following acute right hemisphere stroke. In this left hemisphere stroke population, allocentric neglect was more frequent than egocentric neglect.

Measurement of brain temperature with magnetic resonance spectroscopy in acute ischemic stroke

OBJECTIVE

Pyrexia is associated with poor outcome after stroke, but the temperature changes in the brain after stroke are poorly understood. We used magnetic resonance spectroscopic imaging (water-to-N-acetylaspartate frequency shift) to measure cerebral temperature noninvasively in stroke patients.

METHODS

We performed magnetic resonance diffusion, perfusion (diffusion- and perfusion-weighted imaging), and magnetic resonance spectroscopic imaging, compared temperatures in tissues as defined by the diffusion-weighted imaging appearance (definitely abnormal, possibly abnormal and immediately adjacent normal-appearing brain, and normal brain), and tested associations with lesion and patient characteristics.

RESULTS

Among 40 patients, temperature was higher in possibly abnormal (37.63 degrees C) than in definitely abnormal tissue (37.30 degrees C; p < 0.001) or in normal-appearing brain (ipsilateral, 37.16 degrees C; contralateral, 37.22 degrees C; both p < 0.001). Ischemic lesion temperature increased before normal brain temperature. Higher temperatures occurred in lesions that were large, had diffusion/perfusion-weighted imaging mismatch, had reduced cerebral blood flow, and in clinically severe strokes. Only 1 of 25 patients with ischemic lesion temperature greater than 37.5 degrees C was pyrexial.

INTERPRETATION

Temperature is elevated in acutely ischemic brain. More work is required to determine whether raised temperature results from ischemic metabolic reactions, impaired heat exchange from reduced cerebral blood flow, or early inflammatory cell activity (or a combination of these), but magnetic resonance spectroscopic imaging could be used in studies of temperature after brain injury and to monitor interventions.

Imaging the penumbra in acute stroke

Imaging continues to have a huge impact on the understanding of the ischemic penumbra and the management of acute stroke. Determinants of penumbral tissue fate, such as age, hyperglycemia, hematocrit, and oxygen concentration, are increasingly being recognized using neuroimaging. The significance of the penumbra in the white matter and in posterior circulation stroke is also becoming clearer. Neuroimaging is also making invaluable contributions to clinical decision making in acute stroke, especially in relation to reperfusion therapies in the 3- to 6-hour time window. Despite ongoing questions over the choice of parameters to identify the penumbra and their respective clinical usefulness, imaging is gaining widespread use in acute stroke management. However, definitive evidence of its benefit is still lacking. This review explores the recent progress and controversies relating to imaging of the penumbra.

Magnetic resonance imaging profiles predict clinical response to early reperfusion: the diffusion and perfusion imaging evaluation for understanding stroke evolution (DEFUSE) study

OBJECTIVE

To determine whether prespecified baseline magnetic resonance imaging (MRI) profiles can identify stroke patients who have a robust clinical response after early reperfusion when treated 3 to 6 hours after symptom onset.

METHODS

We conducted a prospective, multicenter study of 74 consecutive stroke patients admitted to academic stroke centers in North America and Europe. An MRI scan was obtained immediately before and 3 to 6 hours after treatment with intravenous tissue plasminogen activator 3 to 6 hours after symptom onset. Baseline MRI profiles were used to categorize patients into subgroups, and clinical responses were compared based on whether early reperfusion was achieved.

RESULTS

Early reperfusion was associated with significantly increased odds of achieving a favorable clinical response in patients with a perfusion/diffusion mismatch (odds ratio, 5.4; p = 0.039) and an even more favorable response in patients with the Target Mismatch profile (odds ratio, 8.7; p = 0.011). Patients with the No Mismatch profile did not appear to benefit from early reperfusion. Early reperfusion was associated with fatal intracranial hemorrhage in patients with the Malignant profile.

INTERPRETATION

For stroke patients treated 3 to 6 hours after onset, baseline MRI findings can identify subgroups that are likely to benefit from reperfusion therapies and can potentially identify subgroups that are unlikely to benefit or may be harmed.

Translational vehicles for neuroprotection

This review will analyse particular criteria in the analysis of stroke diagnosis and treatment, which are pivotal for the successful translation of experimental data from the laboratory to humans.

Imaging of cerebral blood flow and metabolism

PURPOSE OF REVIEW

To review the techniques for imaging cerebral blood flow and metabolism following injury to the brain.

RECENT FINDINGS

Xenon enhanced computerized tomography (Xenon CT), CT perfusion and single photon emission CT provide measurements of cerebral perfusion, while positron emission tomography (PET), and magnetic resonance imaging and spectroscopy (MRI and MRS) are able to assess both perfusion and cerebral metabolism. Xenon CT and CT perfusion are readily available and have proved useful in a variety of causes of brain injury. PET is an extremely useful research tool for defining cerebral physiology, but is limited in its availability. Despite the continuing development of MRI and MRS imaging, the scanning environment remains hostile for critically ill patients, and further research is required before the techniques become generally available.

SUMMARY

Imaging of cerebral blood flow and metabolism has been shown to be useful following a variety of causes of brain injury, as it can help to define the cause and extent of injury, identify appropriate treatments and predict outcome. Imaging based on CT techniques (Xenon CT and CT perfusion) can be implemented easily in most hospital centres, and are able to provide quantitative perfusion data in addition to structural images.

Imaging of acute stroke

Thrombolytic therapy has led to a higher proportion of patients presenting to hospital early, and this, with parallel developments in imaging technology, has greatly improved the understanding of acute stroke pathophysiology. Additionally, MRI, including diffusion-weighted imaging (DWI) and gradient echo, or T2*, imaging is important in understanding basic structural information--such as distinguishing acute ischaemia from haemorrhage. It has also greatly increased sensitivity in the diagnosis of acute cerebral ischaemia. The pathophysiology of the ischaemic penumbra can now be assessed with CT or MRI-based perfusion imaging techniques, which are widely available and clinically applicable. Pathophysiological information from CT or MRI increasingly helps clinical trial design, may allow targeted therapy in individual patients, and may extend the time scale for reperfusion therapy.

Recovery from ischemic stroke: a translational research perspective for neurology

Ischemic stroke is the most frequent neurological disease, characterized by an age-related incidence and chronic disability in the majority of patients. A great challenge in acute stroke is to predict the degree to which a patient will eventually recover. Magnetic resonance imaging has revealed that treatment-induced reperfusion limits the extent of ischemic brain damage, thereby enabling rapid and profound recovery. Nevertheless, patients may retain deficits in motor, sensory or cognitive functions due to the residual lesion. Functional neuroimaging and transcranial magnetic stimulation have shown that recovery is associated with abnormal activation in the perilesional vicinity and in brain areas remote from the lesion. This is likely related to altered functional properties or morphological changes in both cerebral hemispheres. Recent neurorehabilitative strategies, including forced use, mental imagery and peripheral nerve or cortex stimulation, aim at modulating these functional networks. Accordingly, translational research has provided new vistas on the neurobiological mechanisms of recovery and opened future avenues for science-based pharmacological and neurophysiological training strategies in stroke.

Frequency and temporal profile of poststroke hyperglycemia using continuous glucose monitoring

OBJECTIVE

Poststroke hyperglycemia (PSH) is common and has adverse effects on outcome. In this observational study, we aimed to describe the frequency and temporal profile of PSH using a continuous glucose monitoring system (CGMS) in patients with and without diabetes.

RESEARCH DESIGN AND METHODS

Fifty-nine patients with acute hemispheric ischemic stroke were prospectively studied with the CGMS, regardless of medication, admission plasma glucose value, and diabetes status. The CGMS records interstitial glucose every 5 min for 72 h.

RESULTS

On admission, 36% of patients had preexisting diabetes. At the earliest analyzed time point of 8 h from stroke onset, 50% of nondiabetic subjects and 100% of diabetic patients were hyperglycemic (> or =7 mmol/l). This early-phase hyperglycemia was followed by a decrease in glucose 14-16 h poststroke when only 11% of nondiabetic and 27% of diabetic patients were hyperglycemic. A late hyperglycemic phase 48-88 h poststroke was observed in 27% of nondiabetic and 78% of diabetic patients. Thirty-four percent of nondiabetic and 86% of diabetic patients were hyperglycemic for at least a quarter of the monitoring period. Multivariate regression analysis demonstrated that diabetes, insular cortical ischemia, and increasing age independently predicted higher glucose values.

CONCLUSIONS

Poststroke hyperglycemia is common and prolonged despite treatment based on current guidelines. There are early and late hyperglycemic phases in nondiabetic as well as diabetic patients. Treatment protocols with frequent glucose measurement and intensive glucose-lowering therapy for a minimum of 72 h poststroke need to be evaluated.

The Ischemic Penumbra: A New Opportunity for Neuroprotection

The development of acute stroke therapies has yielded only limited success and many failures in multiple clinical trials. The target of acute stroke therapy is that portion of the ischemic region that is still potentially salvageable, i.e. the ischemic penumbra. Neuroprotective drugs have the potential to prevent a portion of the ischemic penumbra from evolving into infracted tissue and designing trials that target neuroprotective drugs at patients with persistent penumbra should enhance the likelihood of a positive outcome. Currently, diffusion and perfusion MRI has the potential to approximate the location and persistence of the ischemic penumbra and can be used in clinical trials to select appropriate patients for inclusion and to evaluate a meaningful treatment effect. Perfusion CT may also have similar capabilities. Use of these imaging modalities in clinical trials and ultimately in clinical practice will likely help in the development and utilization of novel neuroprotective drugs.

Dose Escalation of Desmoteplase for Acute Ischemic Stroke (DEDAS)

Background and Purpose— Desmoteplase is a novel plasminogen activator with favorable features in vitro compared with available agents. This study evaluated safety and efficacy of intravenous (IV) desmoteplase in patients with perfusion/diffusion mismatch on MRI 3 to 9 hours after onset of acute ischemic stroke.

Methods— DEDAS was a placebo-controlled, double-blind, randomized, dose-escalation study investigating doses of 90 μg/kg and 125 μg/kg desmoteplase. Eligibility criteria included baseline National Institute of Health Stroke Scale (NIHSS) scores of 4 to 20 and MRI evidence of perfusion/diffusion mismatch. The safety end point was the rate of symptomatic intracranial hemorrhage. Primary efficacy co-end points were MRI reperfusion 4 to 8 hours after treatment and good clinical outcome at 90 days. The primary analyses were intent-to-treat. Before unblinding, a target population, excluding patients violating specific MRI criteria, was defined.

Results— Thirty-seven patients were randomized and received treatment (intent-to-treat; placebo: n=8; 90 μg/kg: n=14; 125 μg/kg: n=15). No symptomatic intracranial hemorrhage occurred. Reperfusion was achieved in 37.5% (95% CI [8.5; 75.5]) of placebo patients, 18.2% (2.3; 51.8) of patients treated with 90 μg/kg desmoteplase, and 53.3% (26.6; 78.7) of patients treated with 125 μg/kg desmoteplase. Good clinical outcome at 90 days occurred in 25.0% (3.2; 65.1) treated with placebo, 28.6% (8.4; 58.1) treated with 90 μg/kg desmoteplase and 60.0% (32.3; 83.7) treated with 125 μg/kg desmoteplase. In the target population (n=25), the difference compared with placebo increased and was statistically significant for good clinical outcome with 125 μg/kg desmoteplase ( P =0.022).

Conclusions— Treatment with IV desmoteplase 3 to 9 hours after ischemic stroke onset appears safe. At a dose of 125 μg/kg desmoteplase appeared to improve clinical outcome, especially in patients fulfilling all MRI criteria. The results of DEDAS generally support the results of its predecessor study, Desmoteplase in Acute Ischemic Stroke (DIAS).

Ischemic Thresholds for Gray and White Matter

Background and Purpose— Although gray matter (GM) and white matter (WM) have differing neurochemical responses to ischemia in animal models, it is unclear whether this translates into differing thresholds for infarction. We studied this issue in ischemic stroke patients using magnetic resonance (MR) techniques.

Methods— MR studies were performed in patients with acute hemispheric ischemic stroke occurring within 24 hours and at 3 months. Cerebral blood flow (CBF), cerebral blood volume (CBV), mean transit time (MTT), and apparent diffusion coefficient (ADC) were calculated. After segmentation based on a probabilistic map of GM and WM, tissue-specific diffusion and perfusion thresholds for infarction were established.

Results— Twenty-one patients were studied. Infarction thresholds for CBF were significantly higher in GM (median 34.6 mL/100 g per minute, interquartile range 26.0 to 38.8) than in WM (20.8 mL/100 g per minute; interquartile range 18.0 to 25.9; P <0.0001). Thresholds were also significantly higher in GM than WM for CBV (GM: 1.67 mL/100 g; interquartile range 1.39 to 2.17; WM: 1.19 mL/100 g; interquartile range 0.94 to 1.53; P <0.0001), ADC (GM: 918×10 −6 mm 2 /s; 868 to 975×10 −6 ; WM: 805×10 −6 ; 747 to 870×10 −6 ; P <0.001), and there was a trend toward a shorter MTT in GM (GM 4.94 s, 4.44 to 5.38; WM 5.15, 4.11 to 5.68; P =0.11).

Conclusions— GM has a higher infarction threshold for CBF, CBV, and ADC than WM in patients within 24 hours of ischemic stroke onset. Hence, when assessing patients for potential therapies, tissue-specific rather than whole-brain thresholds may be a more precise measure of predicting the likelihood of infarction.

Extending the Time Window for Thrombolysis: Evidence from Acute Stroke Trials

Data from intravenous tissue plasminogen activator studies have shown rapidly diminishing clinical benefit beyond 3 hours when noncontrast CT is used for treatment triage. Newer trials, such as the Desmoteplase in Acute Ischemic Stroke trial, have now successfully pushed the time window out to 9 hours using the concept of penumbral imaging and treatment of the perfusion-diffusion mismatch. Advanced imaging with CT or MR imaging protocols is providing a means for rational physiologic selection and outcomes assessment in stroke treatment.

Clinical-diffusion mismatch predicts the putative penumbra with high specificity

BACKGROUND AND PURPOSE

Perfusion-diffusion (PWI-DWI) mismatch may represent the ischemic penumbra. The complexities associated with perfusion-weighted imaging (PWI) have restricted its use. Mismatch between stroke severity, assessed with the National Institutes of Health Stroke Scale (NIHSS), and the volume of the diffusion-weighted imaging (DWI) lesion (clinical-diffusion mismatch; CDM) has been suggested as a surrogate for PWI-DWI mismatch. We compared CDM with PWI and DWI in acute stroke.

METHODS

Seventy-nine hemispheric stroke patients were imaged within 24 hours of symptom onset and subacutely (3 to 5 days). CDM was defined as NIHSS > or =8 and DWI < or =25 mL. DWI lesion and PWI (Tmax+4s) volumes were measured by planimetric techniques. Acute PWI-DWI mismatch was examined as a continuous variable (mismatch volume=PWIvol-DWIvol) and a categorical variable (mismatch=PWIvol-DWIvol/DWIvol x 100>20%). Early infarct expansion was calculated as DWI(subacute vol/DWI(acute vol).

RESULTS

In the 54 sub-6-hour patients, CDM detected PWI-DWI mismatch with a specificity of 93% (95% confidence interval [CI], 62% to 99%), a positive predictive value of 95% (95% CI, 77% to 100%), but a sensitivity of only 53% (95% CI, 34% to 68%). Alternate DWI and NIHSS cutpoints did not improve test performance characteristics. In addition, subacute DWI expansion was significantly greater in patients with CDM (P=0.01) compared with those without.

CONCLUSIONS

CDM (NIH > or =8, DWI < or =25 mL) predicts the presence of PWI-DWI mismatch with high specificity and low sensitivity. CDM also predicts DWI expansion. CDM may be a useful selection tool in acute stroke therapies, including thrombolysis.